How to erase duplicate contacts on my iPad?

Your question isn’t quite clear and could mean more than one thing but the solution is probably the same regardless. First a little information. Your iCloud account at iCloud.com is the ‘truth’. That is, it contains the data that should be synced with all your devices. That means two things: 1) to fix duplicate contacts you simply have to fix your iCloud account. 2) your devices need to sync properly with your iCloud account. Oftentimes a device not syncing properly is what causes duplicates.

Fixing your problem: Make sure your devices all sync with iCloud.

1) log into your iCloud account and create a dummy contact. Log out and after a few minutes wait, check Contacts on each device and see if the dummy contact has appeared. Yes? Move on to step 2. No? Move to step 1b

1b) on each device that the dummy contact failed to appeared, go to settings (System Preferences on the Mac), sign out of your Apple account, then log back in. Wait a few minutes and check Contacts again. On rare occasions I’ve found it necessary to sign out of Apple on all my devices to get them all to sync properly.

2) syncing is a two way street and sometimes iCloud can send data to your devices but one or more devices won’t send data to iCloud. On each device create a dummy contact and check to see if it appeared online on your iCloud account. For any device that didn’t sync, do step 1b.

3) delete all the dummy contacts. Hope this solves your problem.